THE RESPONSE TO DIFFERING SELECTION ON PLANT PHYSIOLOGICAL TRAITS: EVIDENCE FOR LOCAL ADAPTATION
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Understanding adaptive evolution to differing environments requires studies of genetic variances, of natural selection, and of the genetic differentiation between populations. Plant physiological traits such as leaf size and water-use efficiency (the ratio of carbon gained per water lost) have been suggested by physiological plant ecologists to be important in local adaptation to environments differing in water availability. In this study, I raised families of Cakile edentula var lacustris derived from a wet-site population and a dry-site population in a common greenhouse environment to determine the degree of genetic differentiation between the two populations and the genetic architecture of the traits. The dry-site population had significantly smaller leaf size and significantly greater water-use efficiency than the wet-site population. I used a retrospective selection analysis to compare long-term selection inferred from these results to measures of phenotypic selection from a field experiment. Both direct measures in the field and the retrospective selection gradients were consistent with the hypothesis that greater water-use efficiency and smaller leaves were adaptive in drier environments. Though the correlation between population means for water-use efficiency and leaf size was negative, the genetic correlation within populations between water-use efficiency and leaf size was positive and thus would be expected to constrain the evolutionary response to selection.